June 15 , 2016

by Duane Nash

Making dromaeosaurids nasty again... Yes, there is a bit of a straw man argument in there because many might say they never stopped being nasty. But it is my straw man to make, tear apart, refashion, and burn to smithereens as I will... so while reading these posts on dromaeosaurids always keep in mind the tug of war between past and present interpretations scientific, artistic and popular. Don't forget as well these animals had a long tenure as small to medium sized predators so there is a lot of room for variation in terms of behavior, physiology, and appearance.

Something has happened to dromaeosaurids. They have went from jumping on the back of giant ornithopods to jumping on the back of opossums. They no longer look like the scaly, crazed, methed out gang overlords of the Mesozoic bestiary but instead dapper, attractive fashionistas that glided out of some Mesozoic audobon photo shoot. The psycho reptoid wolf-lizard of the Mesozoic is now the camera friendly fashion model of the Mesozoic.

First things first, the claw. Dat claw. Second to ol' sexy rexy and his chiseled and ruggedly masculine good looks the "killing claw" of dromaeosaurids and especially Deinonychus is probably one of the most emblematic and iconic elements of the theropod predatory arsenal. Are we focusing too much on the "killing claw" in terms of prey dispatch but negating other aspects such as the hand claws and teeth?

A brief review of several of the more pertinent works that have brought us to where we are now.

The Slashing Claw Denied...

While the youngins today have grown up knowing that the killing claw of these animals did not cut scythe like slashes through the hides of dinosaurian megaherbivores I do have to admit to feeling a pinge of let down in my inner fanboi when this was first revealed via the study "Dinosaur killer claws or climbing crampons" way back in 2005. Essentially what Manning et. al. did was build a robotic hydraulic Deinonychus antirrhopus claw and lower leg and attempted to drive it through a pig carcass. Instead of the meter long slashes of lore the results were a little underwhelming. The claw did puncture the carcass but as they attempted to drive it through the flesh tissue simply bunched together below the entry preventing a long slash wound.

Jumping on the Flanks of Giant Herbivores Denied...

Seems pretty legit right?

Well the next paper seemingly expunges the use of the claw in climbing onto the back of megarherbivores. Enter the Raptor Prey Restraint model (RPR) which probably needs little introduction to most readers here. Fowler et. al. reject the notion of dromaeosaurids latching onto the flanks of large herbivores and slashing at prey with foot claws or teeth. Instead prey subequal to the size of the dromaeosaur is pinned down beneath the weight of the predator; stability flaps of the "wings" and movement of the beam like tail help maintain an upright position; and prey is essentially eaten alive if it is not killed outright by the feet.

So ground hawk Deinonychus... what is not to love?

There are some caveats to this model that the authors address that I think warrant repeating:

So, despite the arguments in favor the RPR model for dromaeosaurids and the inherent attractiveness of the model it appears that dromaeosaurids were not >as good at it< as their modern avian counterparts. I am not saying RPR is not a thing and, again, this is from their own paper but it appears that a 50 kg Deinonychus is not equal to a scaled up 50 kg red-tailed hawk in terms of relative grasping power of the feet. Furthermore if you look at the vice like grip that modern raptors can enact in which digit I is rotated completely opposite the other three digits it appears that Deinonychus and other dromaesaurids do not fully rotate the digit opposite the other three for a truly powerful vice like grip.

Again, not trying to imply that RPR is not a thing for dromaeosaurids or that they did not pin and even kill prey with their feet but let us be clear with what the science shows as of now. In dromaeosaurids the RPR model is a relatively less powerful and less efficient version than modern raptors essentially due to the anatomical concessions of retaining some cursorial ability. But I am sure it was still unpleasant...

Actually what dromaeosaur foot graspers remind me of is cats claws and paws (gasp!! mammals). No really... anyone who has had a cat "knead" on them (i.e. breadmaking) knows what I am talking about. As the cat's claws clench shut in a semi-opposable fashion the claws actually pin stuff against the lower arm. In dromaesaurids grasping things would be pinned against the bottom of the metatarsus. This is still a pretty good grip and with both legs working together likely very efficient.

I am obviously not the first to ponder this and it is a perplexing issue in many, if not most, predatory theropods - but what were the clawed wing/arms of dromaeosaurids good for anyway? Fowler et. al. posit them as dynamic stabilizers that combined with the fully feathered tails (like Archaeopteryx not Caudipteryx plumes) allowed these predators to maintain vertical superiority via flapping over prey caught in the foot claws. I like this a lot. So please don't misquote me when I argue that there is something to augment this method not completely replace it.

Those big hand claws seem like such a waste if all they did with their arms is use them in stability flapping.

Issues have been raised as far as the practicality in using the hand claws to grasp small prey dexterously or even bring the hands together to grasp small prey with precision. I honestly don't know what the current thinking on this issue is - or if there is any consensus at all? I would love to hear thoughts and input in the comments section... From what I gather on my cursory research things seem a little equivocal. The wikipedia page on Deinonychus mentions papers that support grasping and others that bring forth some practical questions. Some observations that cause me to question fine tune grasping of small prey items include the potential issue of the "wings" getting in the way of each other when brought together. Also and this is my general observation of patterns in clawed grasping animals: it seems reasonable that in order to achieve a powerful and concise grip that the digits and claws work better aligned in a similar plane. Essentially when you look at the grasping claws of a felid, modern avian raptor, or hell... even our own hands what you see is the digits not varying tremendously in terms of length and that they line up together relatively closely when clenched.

This was simply not the case in maniraptorans with extremely divergent digit length. Such spindly, long claws just seem a little less than ideal for enacting a powerful and concise grip on something small that needs precision. Gripping a tree trunk or "bear hugging" an animal that is fairly large yes... but dextrous grabbing and manipulation of small stuff, I don't buy it. Especially with all those feathers in the way. Could hands like the ones below deftly grab and pick up say a scurrying lizard or mammal?

For these and other reasons I find "fine tune grasping of prey" hypothesis more than wanting. Enough so that other hypotheses warrant exploration.

The hypothesis I will offer - not sure if this idea has been explored yet anywhere to tell you the truth - is going to highlight exaptation of the flight stroke and musculature of the maniraptoran arm to a high degree. This is consistent with the strong hypothesis put forth by Gregory S. Paul that dromaeosaurids are secondarily flightless.

The clawed wing arms of dromaeosaurids could potentially act as clawed battering tools that would further bludgeon, wound, and traumatize prey and/or competitors especially that have been pinned by the feet.

Why not? The arms were strong, long, and heavily clawed after all. More so modern birds just love to bludgeon and smack other things around with their wings. Made famous in a series of posts at Tet Zoo(here 1, part 2, part 3, wrestling birds) by Darren Naish, some wings even have weaponized claws, clubs, and spurs. Dromaeosaurids - likely being secondarily flightless - already had the exaptation to use their wings as bludgeoning tools. All the musculature was already set up for it.

As I mentioned earlier the study by Fowler argued quite well that dromaeosaurids were not >as good< as modern raptors in terms of prey dispatch via the foot claws. Other tools might be needed for prey dispatch... Why not use those nice hand claws powered by the incipient flight stroke to further gouge, pummel, and weaken prey that is being grasped by the footclaws and jaws? Not saying stability flapping did not happen just that stability flapping used in conjunction with with wing pummeling might have some merit. Furthermore the need to maintain vertical position over prey might be just a tad overstated - don't forget the fighting dinosaurs!! After all dromaeosaurids had a little bit more liberty in terms of getting down and dirty on the mat as opposed to accipterids which always have to be mindful of getting grounded with a serious wing injury.

Or the above video which actually doesn't feature a stork eagle fight but loads of domestic breeds battling one another (uuurgh there Nash goes with domestics again). Loads of fleshy caruncled faces, face biting, wing pummeling, talon thrusting. In short very awesomebro!! but not necessarily without merit just because it is awesomebro!! You see the pattern - lunges and strikes are made with the head and/or feet. When brought into the line of fire of the wings, pummeling commences.

This methodology of prey capture/combat is essentially a bit of an inverse of a common tactic used by felids. Anyone who has a pet cat (or who has a cat that has them?) should be familiar with it. If not simply stroke the vulnerable belly of a said felid and the forelimbs and/or jaw will lock into your arm and the back legs commence clawed kicks. In dromaeosaurids it would be the hindlimbs and jaws locking prey into place and the forelimbs delivering blows and trauma via wing pummeling.

In addition to the analogy to modern bird wing pummeling the analysis of theropod stress fractures and forelimb avulsions (Rothschild et. al., 2001) came up with some interesting results with regards to Deinonychus limb use: 43 hand bones and 52 foot bones were examined for signs of stress fracture - none were found. However the second phalanx in the second toe (the killing claw) has a healed fracture (YPM 5205). Why is this important? Well it suggests that the hand claws are not hooking into things and holding them tightly such as appears to be the case with Allosaurus which shows multiple manual pathologies - but the healed killing claw suggests that digit II is hooking into and holding struggling prey/combatants to a higher degree. On the other hand it is worth asking why wing pummeling would not create stress fractures? Perhaps with the force being distributed across the whole surface of the wing (including the feathers) stress fractures would not be such a problem? Are stress fractures a problem in modern birds that wing pummel? Could be an avenue of exploration...

Really I am quite surprised that wing pummeling in dromaeosaurids (and other winged dinos/maniraptorans) has not been proposed before... I mean has it? I dunno, can't find any mention and it seems like a pretty logical inference from what I have gathered.

Two potential criticisms I want to address:

"Yes but modern birds use this wing pummeling in antagonistic disputes not predatorial. Birds of prey do not batter their prey with their wings."

True. Remember Fowler quite convincingly argued that dromaeosaurids were not as relatively >strong< in grasping as accipterids. Does this imply that they were small game specialists? I think not. My contention is bolstered by the "fighting dinosaur" specimen. You will hear some researchers try and explain this situation away as a "rare" or "aberrant" exception to the "baby killer specialist" or "small prey only" model. With all due respect I think that they are mistaken. Not that loads of baby dinosaurs were not munched on, merely that dromaeosaurids and most theropods were not "specialized" for that task.

Given that dromaeosaurid foot grasping was meh compared to modern birds of prey but they were still getting into the thick of things with some pretty rugged combat other lines of attack should be invoked. Those big hand claws seem awfully put to waste in mere stability flapping. Especially when attacking strong retaliatory prey like protoceratopsids.

"Why don't birds of prey pummel prey with their wings?" As I alluded to earlier accipterids differ from dromaeosaurids in that they are dependent on flight. Wing pummeling for them may be selected against as a predatory strategy because they risk an injury, not to mention their feet do the job just fine.

"What about the claws getting stuck in the flesh and skin of the prey during pummeling. Could that be a problem?"

I don't think so. If the downstroke can enmesh the claw in the animal then the upstroke can pull them back out. If claws getting stuck in stuff was so much a problem I guess that implies these animals could not use their claws to climb and clamber in trees as well, because their claws would get stuck in wood, amirite?

Given that; dromaeosaurids are likely secondarily flightless; that their digit morphology is aberrant from other predatory "graspers"; that wings may have got in the way of grasping, especially of objects on the ground; that the fingers remain spread during flexion; that one handed clutching of objects to the chest is just weird; that the elbow can not be fully extended and forelimb mobility is limited; that extant aves often use their wings - sometimes coupled with knobs, spines, and claws - to pummel other animals; for these reasons I posit the hypothesis that clawed wing pummeling is a promising tactic used in dromaeosaurid predatory, combative, and defensive endeavors.

At the moment I see no reason that the idea of wing pummeling can not be extended to other winged dinosaurs and maniraptorans. Hello wing pummeling Gallimimus, Gigantoraptor, Therizinosaurus, and Deinocheirus!!

References

Fowler DW, Freedman EA, Scannella JB, Kambic RE (2011) The Predatory Ecology of Deinonychus and the Origin of Flapping in Birds. PLoS ONE 6(12): e28964. doi:10.1371/journal.pone.0028964

Manning, PL, Payne, D, Pennicott, J, Barrett, PM, Ennos, RA (2006) Dinosaur killer claws or climbing crampons. Biology Letters (2006) 2 110-112 pdf

Rothschild, B. Tanke, D, Ford, TL (2001) Theropod stress fractures and tendon avulsions as a clue to activity. Mesozoic Vertebrate Life. editor Tanke, D & Carptenter, K. Indiana University Press pp331-336

Senter, Phil (2006) Comparison of forelimb function between Deinonychus and Bambiraptor (Theropoda: Dromaeosauridae). JVP Volume 26 Issue 4 2006